In recent years, laminated piezoelectric ceramics in which a piezoelectric ceramic and a metal constituting an internal or external electrode are laminated have begun to be employed as piezoelectric actuators and piezoelectric transducers for controlling spraying of color ink in ink-jet printers and controlling the spraying of common rail fuels in diesel engines. In such piezoelectric ceramics, PZT materials comprised primarily of composite oxides of lead, zirconium, and titanium are employed to achieve high elongation (d33) with the application of voltage.
Examples of products employing these PZT materials are piezoelectric actuators, piezoelectric vibrators in the form of computer oscillators, ceramic filters, piezoelectric transducers, the ignition elements of gas instruments, piezoelectric buzzers, ultrasonic transceivers, microphones, and ultrasonic humidifiers. Among these, piezoelectric actuators, which are solid-state elements employing voltage effects, consume less power, respond more rapidly, and permit finer positional control than conventional electromagnetic actuators configured of a coil wound around a magnetic member. They also have the advantages of generating little heat and being small in size and weight.
The piezoelectric material employed in piezoelectric actuators must afford high voltage characteristics with regard to change in temperature, durability, and the like. Further, various laminated piezoelectric actuator applications of laminated ceramic capacitor technology have recently been developed as small actuators yielding high strain at relatively low applied voltages. High voltage characteristics are naturally also demanded of these laminated piezoelectric actuators.
The above-mentioned actuators require the application of a relatively high voltage. Thus, good conductors such as silver-palladium alloy (Ag—Pd alloy) that generate little heat are widely employed as the electrodes in piezoelectric elements. The piezoelectric elements for these laminated piezoelectric actuators are generally manufactured by coating an internal electrode-forming conductive paste on a ceramic green sheet of piezoelectric material, laminating multiple layers, and conducting simultaneous sintering. That is, in the manufacturing of a laminated piezoelectric actuator, since the internal electrode and the piezoelectric material are sintered simultaneously, a material capable of being sintered at low-temperature must be employed so that the internal electrode does not melt at the sintering temperature.
To adequately sinter the laminated piezoelectric ceramic employing the above-described PZT ceramic, sintering must be conducted at a temperature of greater than or equal to 1,100° C. in an oxidizing atmosphere comprising oxygen or the like. In the conventional method of manufacturing laminated piezoelectric ceramics, a paste primarily comprising a metal is coated on a green sheet or sheet-like formed member of PZT ceramic powder to form a conductive layer, these are laminated, the binder is removed, and sintering is conducted at high temperature. That is, the mainstream method is that of forming an integrated member by simultaneous sintering. Thus, the metal employed in lamination is limited to a noble metal comprising platinum group, such as a silver-palladium alloy, that does not melt in high-temperature oxidizing environments and has a melting point of greater than or equal to 1,100° C.
Generally, the melting point of this silver palladium alloy increases with the palladium content in the alloy. Further, since silver diffuses into the piezoelectric material in high temperature sintering, the durability as an actuator drops precipitously. Since palladium is expensive, it is desirable to reduce to the extent possible the quantity of palladium blended into the alloy to lower the cost of the product.
From these perspectives, silver-palladium alloys incorporating comparatively little palladium have been developed. The Ag70-Pd30 alloy, which is relatively good from both the perspective of heat resistance and cost, is widely employed as an electrode material in piezoelectric elements. Ag70-Pd30 alloy comprises about 30 weight percent (25 to 35 weight percent) palladium and can prevent melting at temperatures below 1,150° C. Piezoelectric elements produced by simultaneously sintering Ag70-Pd30 alloy and a piezoelectric material at 1,150° C. have been reported. However, the piezoelectric characteristics of the piezoelectric elements obtained have not been satisfactory.
Accordingly, attempts have been made to improve piezoelectric characteristics by employing a piezoelectric material that will sinter with silver palladium at temperatures below 1,150° C. An example of such a materials is the PZT piezoelectric material denoted by Pb[(Zn1/3Nb2/3)(Ni1/3Nb2/3),Zr,Ti]O3, which can be employed in laminated piezoelectric actuators employing an Ag70-Pd30 internal electrode. Even when simultaneously sintered with an internal electrode at a temperature (such as 1,120° C.) lower than 1,150° C., this piezoelectric material exhibits some level of piezoelectric characteristics.
However, the piezoelectric characteristics that are achieved, particularly the electromechanical coupling factor (Kp), are still inadequate. Further, a reduction in the amount of palladium blended into the silver-palladium alloy is desired in addition to a reduction in the sintering temperature. However, as of today, no piezoelectric material that can be sintered at a temperature of less than or equal to 1,120° C. and yields good piezoelectric characteristics has been reported.
Nor is any piezoelectric material known in which, even at a high Curie temperature Tc, the elongation during voltage application, particularly the piezoelectric strain constant d33 in the longitudinal direction, is higher than the previous level, while also exhibiting a low dielectric loss factor tan δ.
Thus, the present invention, devised to solve the above-stated problems, has for its object to provide a ceramic material that can be prepared by simultaneous sintering with not only a silver-palladium alloy such as Ag70-Pd30Ag—Pd but also alloys having a lower palladium blend proportion and other metals having low melting points as an electrode—that is, by sintering at a temperature of 950° C. or below—and that has good piezoelectric characteristics, and in particular, good longitudinal elongation (d33) and a low dielectric loss factor (tan δ) even at a high Curie temperature Tc; as well as a ceramic material having a good electromechanical coupling factor (Kp).
A further object of the present invention is to provide laminated piezoelectric elements and piezoelectric elements employing the above ceramic materials.